Ethylene adsorption on Pt/Au/SiO2 catalysts

Ethylene adsorption on a Pt/Au/SiO₂ catalyst (2 wt% Pt; Au/Pt atomic ratio of 10) was studied using adsorption microcalorimetry and FTIR spectroscopy. Ethylene adsorption at 300 K on Pt/Au/SiO₂ produced π‐bonded, di‐σ‐bonded, and ethylidyne species with an initial heat of 140 kJ/mol, compared to a heat of 157 kJ/mol for Pt/SiO₂ on which only ethylidyne species formed. At 203 and 263 K, ethylene adsorbed on Pt as well as on Au surface atoms for the Pt/Au/SiO₂ catalyst. Quantum chemical, DFT calculations indicate that Au exerts a significantly smaller electronic effect on Pt than does addition of Sn to Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Adsorption of nitrogen dioxide on polycrystalline gold

The adsorption of nitrogen dioxide (NO₂) on a polycrystalline Au surface was studied by temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). Three desorption states due to chemisorbed NO₂ were observed using TPD, with desorption activation energies,E _d , of 11,13, and 17 kcal/mol. The desorption energies reflect the heats of adsorption of NO₂ on the polycrystalline gold surface, since NO₂ adsorption is not an activated process. Desorption of physisorbed NO₂ from N₂O₄ multilayers was also seen at 130–140 K. The sticking probability of NO₂ at 120 K is independent of coverage indicating a strong influence of a precursor state in the adsorption kinetics. Vibrational spectra using HREELS show that chemisorbed NO₂ is molecularly adsorbed on the surface, probably as a Au O,O'-nitrito surface chelate. No evidence for the dissociation of NO₂ on Au was found using AES, TPD, or HREELS, even for large exposures of NO₂ at surface temperatures up to 500 K. Comparison of these results with those for NO₂ adsorption on a Au(111) surface is made. High energy sites, such as steps and kinks, and other crystal faces of Au can chemically bond NO₂ more tightly than occurs on Au(111), but the activation energy for dissociation of NO₂ at all of these sites exceeds 17 kcal/mol, and thus NO₂ adsorption is reversible on Au under low pressure conditions.     

First-principles analysis of the hydrogenation of carbon monoxide over palladium

The reaction paths for the hydrogenation of CO to methanol over Pd_x (x = 1–4 and 19) cluster models were examined using first-principle density functional quantum chemical calculations. The predicted adsorption energies for the most favorable binding modes for CO, H₂, HCO, H₃CO, CH₃OH, C, O and H on a Pd₁₉ model Pd(111) clusters were -147, -62, -340, -51, -195, -33, -610, -349 and -251 kJ/mol, respectively. The most favorable modes for CO, CH₃O, H, C and O on Pd(111) were all found to be the 3-fold fcc site. The most favorable modes for the formyl and formaldehyde surface intermediates at low coverage were the 3-fold (ζ²μ³), and the di-σ sites, respectively. At higher surface coverages, however, the atop ζ¹ (C) and the π modes for the formyl and formaldehyde intermediates were more likely. The computed adsorption energies were subsequently used to compute overall reaction energies for the hydrogenation of CO to methanol. The initial hydrogenation of CO to the ζ¹ (C) HCO intermediate was found to be +52 kJ/mol endothermic and has been speculated as a possible rate-limiting step. The remaining surface hydrogenation steps become increasingly more exothermic as more hydrogen was added. The elementary steps of formyl to formaldehyde, formaldehyde to methoxide and methoxide to methanol were computed to be -9, -26 and -33 kJ/mol, respectively. The overall energy for CO dissociation was found to be highly unlikely at +260 kJ/mol and a clear indication that methanation and chain growth chemistry is not very likely over Pd. The most favorable reaction coordinate for the hydrogenation of CO to the ζ¹ (C) formyl intermediate was that which proceeds over a single Pd site where there is a migratory insertion of the CO into a Pd–H bond. The barrier for this path was computed to be +78 kJ/mol on the Pd₁₉ cluster. There was a very weak dependence on cluster size. This is a likely indication that this reaction is structure insensitive. A second path which involved the coupling of H and CO over a bridge site was found to be +130 kJ/mol which is less likely, but may also occur under different conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Numerical estimation of hydrogen storage limits in carbon-based nanospaces

Theoretical limits of the hydrogen adsorption in carbon nanospaces are modeled using Monte Carlo simulations. A detailed analysis of storage capacity of slit pores has been performed as a function of the pore size, gas pressure (up to 100 bars) and temperature of adsorption (77 and 298 K). The H₂-slit wall interaction has been modeled assuming energies of adsorption ranging from 4.5 kJ/mol (pure graphene surface) to 15 kJ/mol (hypothetical chemically modified graphene). The quantum nature of H₂ has been incorporated in the calculations using the Feynman-Hibbs approach. It has been shown that in a hypothetical chemically modified porous carbon, with energy of adsorption of 15 kJ/mol or higher and pore size between 0.8 and 1.1 nm, the gravimetric and volumetric storage capacity can achieve targets required for practical applications. The relation between the energy of adsorption and the effective delivery has been discussed.     

Hydrogen Formation in the Reactions of Methanol on Supported Au Catalysts

The adsorption and reactions of methanol have been investigated on Au metal supported by various oxides and carbon Norit of high surface area. Infrared spectroscopic studies revealed the dissociation of methanol at 300 K, which mainly occurs on the oxide-supports yielding methoxy species. The presence of Au already appeared in the increased amounts of desorbed products in the TPD spectra. The reaction pathway of the decomposition and the activity of the catalyst sensitively depend on the nature of the support. As regards the production of hydrogen the most effective catalyst is Au/CeO₂ followed by Au/MgO, Au/TiO₂ and Au/Norit. In contrast, on Au/Al₂O₃ the main process is the dehydration reaction yielding dimethyl ether. On Au/CeO₂ the decomposition of methanol starts above ~500 K and approaches total conversion at 723–773 K. The products are H₂ (~68%) and CO (~27%) with very small amounts of methane and CO₂. The decomposition of methanol follows the first order kinetics. The activation energy of this process is 87.0 kJ/mol. The selectivity of H₂ formation at 573–773 K was ~90%, this value increased to 97% using CH₃OH:H₂O (1:1) reacting mixture indicating the involvement of water in the reaction. No deactivation of Au catalysts was experienced at 773 K in ~10 h. It is assumed that the interface between Au and partially reduced ceria is responsible for the high activity of Au/CeO₂ catalyst.     

Separation of CH4/N2 by an ultra-stable metal–organic framework with the highest breakthrough selectivity

Selectively separating CH₄ from N₂ in natural gas purification is extremely important, but challenging. Herein, a copper-based metal–organic framework (MOF) NKMOF-8-Me with inert pore environment was reported for efficient CH₄/N₂ separation. Adsorption results show that this material owns the highest CH₄ uptake (1.76 mmol/g) and initial adsorption heat (Q_st⁰) of CH₄ (28.0 kJ/mol) as well as difference in Q_st⁰ (9.1 kJ/mol) among all materials with good water stability. Breakthrough experiments confirm that this MOF can completely separate the CH₄/N₂ mixture with the highest CH₄/N₂ breakthrough selectivity (7.8) reported so far. Theoretical calculations reveal the separation mechanism is the short average distance between CH₄ and pore wall, resulting in a stronger adsorption affinity for CH₄. In addition, this MOF exhibits highly structural stability and regeneration. These results guarantee this MOF as a promising adsorbent for the recovery of CH₄ from coalbed methane.     

Studies on cobalt catalyst supported on silica with different pore size for Fischer–Tropsch synthesis

The adsorption and surface reactions of acetonitrile and acetonitrile-oxygen gas mixture were studied on TiO₂-supported Au catalysts at 300–673 K. FTIR spectra show different kinds of molecularly adsorbed CH₃CN:acetonitrile can be bonded to weak Lewis acid sites (2295 cm⁻¹), to strong Lewis acid sites (2337 cm⁻¹) of titania; it can be coordinated linearly through the lone electron pair of the N atom on Au sites and η² (C,N) CH₃CN species can be formed on Au particles. CH₃CN dissociates on Au sites, the resulting CN_(a) can be oxidized in small extent by lattice oxygen and in a greater extent by gaseous oxygen into NCO surface species. The formation of other products (CH₃NH₂, H₂, CO₂, CH₄, C₂H₄ and CO) was demonstrated and discussed.     

Formation of N2O and (NO)2 During NO Adsorption on Au 3D Crystals

The adsorption of NO on Au 3D hemispherical crystals (field emitter tips) has been studied by means of pulsed field desorption mass spectrometry (PFDMS) under dynamic gas flow conditions and at 300 K. Local chemical probing of ~200 Au sites in the stepped surface region between the central (111) pole and the peripheral (001) plane leads to the detection of NO⁺, N₂O⁺ and (NO) species. Obviously, molecular NO adsorption on stepped Au surfaces can lead to dimerization. Nitrous oxide formation probably occurs via the dimer, (NO)₂.     

Calculated energies of adsorption of non-hydrocarbon species on diamond H/C(1 1 1) surface and the abstraction energies of these species abstracted by hydrogen atoms using ab initio calculation

The energetics of adsorption of non-hydrocarbon radical species on H/C(1 1 1) diamond surface and the abstraction energies of these species abstracted by hydrogen atoms, which are in excess in gas phase in the diamond thin film growth using the chemical vapor deposition (CVD) method, were examined using ab initio calculation method. Based on the calculated results for the examined species, which include H, F, OH, NH₂, Cl, CH_mX_n (X=F or Cl) radicals, the tendency of incorporation of F, O, N, H and Cl atoms in the diamond thin film is discussed. The high adsorption energy and the high abstraction energy abstracted by excess gas-phase H atoms for F radicals suggest that F atom has the highest tendency to stay in the diamond thin film among the examined non-carbon atoms. In contrast, the comparable adsorption energy of Cl atom with other examined radicals except F radical, and its low abstraction energy, indicate that Cl atom possesses the least tendency to be incorporated in the diamond thin film. For O, N and H atoms, their calculated abstraction energy values suggest that the overall order of tendency of incorporation in diamond thin film is F>O>N>H>Cl. In addition, the energetically comparable adsorption energy for the CH₂Cl radical, compared with the other examined CH_mX_n species, and the low abstraction energy of Cl atom support that CH₂Cl is a good growth species in diamond CVD thin film growth.     

Reactivity of Ethyl Groups on a Sn/Pt(111) Surface Alloy

In order to probe the thermal stability and reactivity of ethyl intermediates on Pt-Sn alloy catalysts, we have synthesized these species by reaction of H atoms with adsorbed ethylene on a ( ) R30°-Sn/Pt(111) surface alloy. Adsorbed ethyl groups are stable until 376 are they react to evolve ethane, ethylene, and H₂. The activation energy for ethyl dehydrogenation is E_dehyd* 97 kJ mol, which is twice that reported on Pt(111). In addition, we place a lower limit of E_dehyd* 97 kJ mol on the barrier to ethyl hydrogenation on this alloy.     

Syntheses and characterization of polystyrene‐supported 2,5‐dimercapto‐1,3,4‐thiodiazole and its sorption behavior for Pd(II), Pt(IV), and Au(III)

A type of chelating resin crosslinking polystyrene‐supported 2,5‐dimercapto‐1,3,4‐thiodiazole (also called bismuththiol I, BMT), containing sulfur and nitrogen atoms, was prepared. The structure of PS‐BMT was confirmed by FTIR, elemental analysis, and X‐ray photoelectron spectroscopy (XPS). Adsorption of Pd(II), Pt(IV), and Au(III) was investigated. The capacity of PS‐BMT to adsorb Pd(II) and Pt(IV) was 0.190 and 0.033 mmol/g, respectively. The adsorption dynamics of Pd(II) showed that adsorption was controlled by liquid film diffusion and that the apparent activation energy, E_a, was 32.67 kJ/mol. The Langmuir model was better than the Freundlich model in describing the isothermal process of Pd(II), and the ΔG, ΔH, and ΔS values calculated were ?0.33 kJ/mol, 26.29 kJ/mol, and 87.95 J mol^?1 K^?1, respectively. The mechanisms of adsorption of Pd(II), Pt(IV), and Au(III) were confirmed by XPS. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 631–637, 2006     

CO2 capture efficiency in post‐combustion using MWNT‐PAA in a packed bed column

The adsorption capacity of polyaspartamide (PAA) and multi‐wall carbon nanotubes with polyaspartamide (MWNT‐PAA) was investigated through a packed bed column with the flowing of flue gas composed of 15 % CO₂, 5 % O₂ and the balance N₂. The adsorption performed at 25 °C, 110 kPa and inlet gas flow rate of 60 mL/min resulted in high CO₂ adsorption capacity of 5.70 and 10.20 mmol‐CO₂/g for PAA and MWNT‐PAA, respectively. The adsorption kinetics was very high, so 7 min were enough for the effluent gas to reach the breakthrough after saturation. The consistency of adsorbents in recurring regeneration was successful through a continuous TSA system of 10 cycle adsorption‐desorption with temperatures of 25–100 °C. The evaluation of heat through differential scanning calorimetry (DSC) resulted in exothermic adsorption with heat release of 45.14 kJ/mol and 124.38 kJ/mol for PAA and MWNT‐PAA, respectively. The heat release was found favourable to promote the desorption as the temperature could rise after adsorption. This is an advantage for energy efficiency, as it depicts the potential of energy recovery. Thus, both adsorbent PAA and MWNT‐PAA were demonstrated to be promising for CO₂ adsorption capture in post‐combustion.     

Aqueous N2O Reduction with H2 Over Pd-Based Catalyst: Mechanistic Insights From Experiment and Simulation

Nitrous Oxide (N₂O), an ozone depleting greenhouse gas, is an observed intermediate in aqueous nitrate/nitrite reduction mediated by both natural microbial and synthetic laboratory catalysts. Because of our interest in catalytic nitrate/nitrite remediation, we have endeavored to develop a detailed concordant experimental/theoretical picture of N₂O reduction with H₂ over a Pd catalyst in an aqueous environment. We use batch experiments in H₂ excess and limiting conditions to examine the reduction kinetics. We use density functional theory (DFT) to model the elementary steps in N₂O reduction on model Pd(100), Pd(110), Pd(111) and Pd(211) facets and including the influence of adsorbed O, H, and of H₂O. Both experiments and theory agree that hydrogen is necessary for removal of adsorbed oxygen from the catalyst surface. The dissociation of N₂O to N₂(g) and O(ads) is facile and in the absence of H proceeds until the catalyst is O-covered. Water itself is proposed to facilitate the hydrogenation of surface O by transferring absorbed hydrogen to Pd-absorbed O and OH. We measure an apparent activation energy of 41.4?kJ/mol (0.43?eV) for N₂O reduction in the presence of excess H₂, a value that is within 0.1?eV of the barriers determined theoretically.     

Why Au and Cu Are More Selective Than Pt for Preferential Oxidation of CO at Low Temperature

Self-consistent, periodic density functional theory (DFT) calculations and micro-kinetic modeling are used to compare selectivity for the preferential oxidation of CO (PROX) with respect to H₂ based on studies of elementary reaction steps on the (111) facet of Au, Cu and Pt. The first step of H oxidation (OH formation) has a higher activation barrier than the second step (H₂O formation) on all three metal surfaces, indicating that OH formation competes with CO oxidation for the removal of trace amounts of CO from a typical reformate gas. The activation energy barrier for CO oxidation is found to be 0.18eV on Au(111), 0.82eV on Cu(111) and 0.96eV on Pt(111), whereas the barrier for OH formation is 0.90, 1.28 and 0.83eV respectively. A micro-kinetic model based on the DFT results shows that trends in the selectivity of these metals at different temperatures is due to (i) differences in the rate constants of the competitive CO and H oxidation reactions, and (ii) differences in the CO and H surface coverages. Our results explain why Au and Cu are more selective PROX catalysts compared to Pt at low temperatures. At higher temperatures, Pt and Cu lose some of their selectivity to CO oxidation, whereas the selectivity on Au decreases substantially primarily because of the significantly weaker CO adsorption.     

The kinetics of CO oxidation by adsorbed oxygen on well‐defined gold particles on TiO2(110)

Very tiny Au particles on TiO₂ show excellent activity and selectivity in a number of oxidation reactions. We have studied the vapor deposition of Au onto a TiO₂(110) surface using XPS, LEIS, LEED and TPD and found that we can prepare Au islands with controlled thicknesses from one to several monolayers. In order to understand at the atomic level the unusual catalytic activity in oxidation reactions of this system, we have studied oxygen adsorption on Au/TiO₂(110) as a function of Au island thickness, and have measured the titration of this adsorbed oxygen with CO gas to yield CO₂, as function of Au island thickness, CO pressure and temperature. A hot filament was used to dose gaseous oxygen atoms. TPD results show higher O₂ desorption temperatures (741 K) from ultrathin gold particles on TiO₂(110) than from thicker particles (545 K). This implies that O_a bonds much more strongly to ultrathin islands of Au. Thus from Brønsted relations, ultrathin gold particles should be able to dissociatively adsorb O₂ more readily than thick gold particles. Our studies of the titration reaction of oxygen adatoms with CO (to produce CO₂) show that this reaction is extremely rapid at room temperature, but its rate is slightly slower for the thinnest Au islands. Thus the association reaction (CO_g + O_a → CO_2,g) gets faster as the oxygen adsorption strength decreases, again as expected from Brønsted relations. For islands of about two atomic layers thickness, the rate increases slowly with temperature, with an apparent activation energy of 11.4 ± 2.8 kJ/mol, and shows a first‐order rate in CO pressure and oxygen coverage, similar to bulk Au(110).     

Atomic and Molecular Adsorption on Re(0001)

Using periodic, self-consistent density functional theory calculations, the adsorption of several atomic (H, S, N, O and C) and molecular (CO₂, N₂, NH₃, HCN, CO and NO) species and molecular fragments (NH₂, NH, CN, CNH₂, HNO, NOH, CH₃, CH₂, CH and OH) on the (0001) facet of rhenium at a coverage of 0.25 ML has been studied. Preferred binding sites with their corresponding binding energy and deformation energy of the surface, as well as an estimated diffusion barrier of each species have been determined. Atomic species and molecular fragments tend to bind to threefold sites, whereas molecular species tend to bind to top sites. The binding strength, with respect to the corresponding gas phase species and in increasing order for all species studied, is: CO₂ < N₂ < NH₃ < CO < CH₃ < HCN < NO < H < NH₂ < OH < CH₂ < CNH₂ < CN < HNO < NH < NOH < S < N < O < CH < C. The vibrational frequencies of all species in their most energetically favorable adsorbed configuration have been calculated. Finally, the thermochemistry of adsorption and decomposition of NO, NO + H, NH₃, N₂, CO₂, CO and CH₄ on Re(0001) has been analyzed.     

Gold nanoparticles on ceria: importance of O vacancies in the activation of gold

Synchrotron-based techniques (high-resolution photoemission, in-situ X-ray absorption spectroscopy, and time-resolved X-ray diffraction) have been used to study the destruction of SO₂ and the water-gas shift (WGS, CO + H₂O → H₂ + CO₂) reaction on a series of gold/ceria systems. The adsorption and chemistry of SO₂ was investigated on Au/CeO₂(111) and AuO _x /CeO₂ surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO₂(111) was 4–7 kcal/mol larger than on Au(111). However, there was negligible dissociation of SO₂ on the Au/CeO₂(111) surfaces. The full decomposition of SO₂ was observed only after introducing O vacancies in the ceria support. AuO _x /CeO₂ surfaces were found to be much less chemically active than Au/CeO₂(111) or Au/CeO_2−x (111) surfaces. In a separate set of experiments, in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy were used to monitor the behavior of nanostructured {Au + AuO _x }–CeO₂ catalysts under the WGS reaction. At temperatures above 250 °C, a complete AuO _x → Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO₂(111) single crystal corroborate that cationic Au^δ+ species cannot be the key sites responsible for the WGS activity at high temperatures. The active sites in {Au + AuO _x }/ceria catalysts should involve pure gold nanoparticles in contact with O vacancies of the oxide.     

Theoretical Insight into the Nature of Ammonia Adsorption on Vanadia-Based Catalysts for SCR Reaction

Density functional theory (DFT) calculations were carried out on monomeric and oligomeric vanadium oxide clusters to probe the factors leading to the formation of NH₄ species from the adsorption of ammonia. The interaction of ammonia with monomeric vanadium oxide clusters leads to the formation of hydrogen-bonded NH₃ species, with energy changes for ammonia adsorption near -50 kJ/mol. The interaction of ammonia with oligomeric vanadium oxide clusters leads to the formation of bidentate NH_4 species, where the ammonium cation is coordinated between two V=O groups on adjacent vanadium cations. The energy change for ammonia adsorption in this mode is near -100 kJ/mol. Adsorption of ammonia as NH₄ species was not observed when the oligomeric vanadium oxide clusters were reduced by addition of hydrogen atoms, i.e., in clusters where the formal oxidation state of the vanadium cations was 4+. Based on our findings, a model for the generation of Brönsted acidity through the interaction of vanadium oxide oligomers with the titanium oxide support is proposed.     

Phosphonates Meet Metal−Organic Frameworks: Towards CO2 Adsorption

Here we report a new highly microporous zirconium phosphonate material synthesized under solvothemal conditions. The specific Brunauer-Emmett-Teller (BET) surface area of the “unconventional metal−organic framework” (UMOF) is measured to be ∼900 m²/g, after following an appropriate activation protocol. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) shows that the material bears a free −OH functionality on the phosphonate linker that may interact with CO₂. CO₂ adsorption isotherms were collected and a measured heat of adsorption of 31 kJ/mol was obtained. In addition, adsorption isotherms of CO₂, N₂, and CH₄ at 298 K combined with Ideal Adsorbed Solution Theory (IAST) show that the material can be expected to display high selectivities for uptake of CO₂ versus N₂ or CH₄.     

Relaxation mechanism in several kinds of polyethylene estimated by dynamic mechanical measurements, positron annihilation, X-ray and C solid-state NMR

Relaxation processes of several kinds of polyethylene films and fibers with different molecular orientational degrees and crystallinities were extensively investigated by the dynamic mechanical relaxation, positron annihilation and ¹³C nuclear magnetic relaxation (¹³C NMR). From complex dynamic tensile modulus, the activation energies of α₁ and α₂ relaxations were determined to be 97-118 and 141-176 kJ/mol, respectively. The activation energy of β relaxation was 114-115 kJ/mol. These values were similar to those of α₁ relaxation reported already. For γ relaxation mechanisms, there existed two mechanisms, γ₁ and γ₂, the activation energies being 9-11 and 23-25 kJ/mol, respectively. The values were independent of the molecular orientation and crystallinity. The two local motions indicate that non-crystalline phase composes of two regions of non-crystalline phase, rubber-like amorphous phase and interfacial-like amorphous phase. From ¹³C NMR measurements of ¹³C longitudinal relaxation time for the non-crystalline phase, the activation energy was 20.7 kJ/mol. This value is close to the activation energy (23-25 kJ/mol) of the γ₂ relaxation estimated by the dynamic mechanical measurement. The result by ¹³C NMR did not provide two kinds of activation energy, indicating combined influence of the two correlation times. Even so, the activation energies obtained by ¹³C NMR indicated that the γ₂ relaxation mainly is due to the motion of the C-C central bond of a short segment (e.g. three to four CH₂) within interfacial-like amorphous phase. The γ and β relaxation peaks by the dynamic mechanical measurements corresponded to the first and second lifetime transition of ortho-positronium indicating, in turn, a drastic change in free volume by local mode relaxation.